MOUNT_FDESC(8) System Manager's Manual MOUNT_FDESC(8)
mount_fdesc – mount the file-descriptor file system
mount_fdesc [-o options] fdesc mount_point
The mount_fdesc command attaches an instance of the per-process file
descriptor namespace to the global filesystem namespace. The conventional
mount point is /dev and the filesystem should be union mounted in order to
augment, rather than replace, the existing entries in /dev. This command
is normally executed by mount(8) at boot time.
The options are as follows:
-o Options are specified with a -o flag followed by a comma separated
string of options. See the mount(8) man page for possible options
and their meanings.
The contents of the mount point are fd, stderr, stdin, stdout and tty.
fd is a directory whose contents appear as a list of numbered files which
correspond to the open files of the process reading the directory. The
files /dev/fd/0 through /dev/fd/# refer to file descriptors which can be
accessed through the file system. If the file descriptor is open and the
mode the file is being opened with is a subset of the mode of the existing
descriptor, the call:
fd = open("/dev/fd/0", mode);
and the call:
fd = fcntl(0, F_DUPFD, 0);
The files /dev/stdin, /dev/stdout and /dev/stderr appear as symlinks to the
relevant entry in the /dev/fd sub-directory. Opening them is equivalent to
the following calls:
fd = fcntl(STDIN_FILENO, F_DUPFD, 0);
fd = fcntl(STDOUT_FILENO, F_DUPFD, 0);
fd = fcntl(STDERR_FILENO, F_DUPFD, 0);
Flags to the open(2) call other than O_RDONLY, O_WRONLY and O_RDWR are
The /dev/tty entry is an indirect reference to the current process's
controlling terminal. It appears as a named pipe (FIFO) but behaves in
exactly the same way as the real controlling terminal device.
mount(2), unmount(2), tty(4), fstab(5), mount(8)
No ~. and .. entries appear when listing the contents of the /dev/fd
directory. This makes sense in the context of this filesystem, but is
inconsistent with usual filesystem conventions. However, it is still
possible to refer to both ~. and .. in a pathname.
This filesystem may not be NFS-exported.
The mount_fdesc utility first appeared in 4.4BSD.
BSD 4.4 March 27, 1994 BSD 4.4